Electrolytic production of high purity alkali metal hydroxide

ABSTRACT

Concentrated alkali metal hydroxide substantially free of alkali metal halide and other impurities is produced by the electrolysis of an alkali metal halide solution in an electrolytic cell having a dimensionally stable anode and a metal cathode separated by an electrically conductive stable selectively permeable hydrated cation ion-exchange membrane film of a fluorinated copolymer having pendant sulfonic acid groups or derivatives of such groups. The membrane film is capable of use at high temperatures and under severely corrosive chemical conditions for extended periods without degradation.

This is a division of application Ser. No. 191,424, filed Oct. 21, 1971now U.S. Pat. No. 4,025,405.

BACKGROUND OF THE INVENTION

(1) Field of the invention

This invention relates generally to a process and apparatus forproducing high purity alkali metal hydroxide in an electrolytic cell.

More specifically this invention relates to a process and apparatus forproduction of concentrated alkali metal hydroxide substantially free ofalkali metal halides and other impurities in an electrolytic cellwherein a stable selectively permeable hydrated membrane is interposedbetween a dimensionally stable foraminous anode and a cathode to formindividual anolyte and catholyte compartments.

(2) State of the art

Concentrated alkali metal hydroxide solutions have previously beenprepared by the electrolysis of alkali metal halide solutions in mercurytype electrolytic cells, such cells frequently being referred to asflowing cathode mercury cells. The alkali metal hydroxide aqueoussolution produced in such cells is generally of high concentration forexample about 50 to 73 weight percent alkali metal hydroxide, andsubstantially free of alkali metal halide. A product with thesecharacteristics directly satisfies the requirements for variousindustrial applications. However, the mercury type cell currently hasseveral disadvantages the major one being the pollution of streamscaused by the effluent of said cells. This discharge has created amercury pollution problem in the environment wherever such cells are inuse. Although extensive efforts have been made to control the amount ofmercury pollution caused by the effluent from such cells, it isgenerally considered that the complete elimination of the pollution ofwater and soil is virtually impossible. Because of the currentobjections to any type of pollution and the very strict governmentalregulatory provisions proposed to control all types of pollution, thereis the imminent possibility that such cells will not be tolerated formore than a few years and soon will become obsolete. Even if improvedmethods of preventing mercury pollution by the effluent of said cellsare found and introduced, and even if governmental regulatory provisionscan be met and the cells remain in use, they suffer the additionaldisadvantages of being expensive and complex, and of frequently causingerratic operating conditions. Also cell operators are constantly exposedto toxic hazards. Large quantities of mercury are required per cell andthe market price of mercury is generally high. In addition a certainquantity of the mercury utilized in the normal operation of such cellsis irretrievably lost in the effluent streams regardless of the rigidrecovery techniques employed to reclaim the mercury from the amalgamformed in the cell.

Diaphragm cells are also known for producing alkali metal hydroxidesolutions electrolytically. In this type of cell a porous cathode withan overlying porous diaphragm is used to separate or to serve as abarrier between the catholyte and anolyte compartments of the cell. Anobjectionable feature of this cell in the electrolysis of alkali metalhalide is the porosity of the diaphragm which, although it serves toseparate the cell into anode and cathode compartments, permits theaqueous electrolyte solution to be unselectively transported into thecathode compartment. Because of the water content of the electrolytesolution the concentrations of the alkali metal hydroxide product arelimited to from about 12 to about 18 weight percent. Anotherdisadvantage of this cell is the tendency of the hydroxyl ion formed inthe cathode compartment to migrate back through the porous cathode anddiaphragm because of electromigration and diffusion. This so calledback-migration results in undesirable side products or impurities and aloss of operating current efficiency in the cell because of theadditional current consumed by the cathode-anode anion migration. Priorart efforts to overcome the disadvantage of hydroxyl ion back-migrationhave resulted in forming a flow of the alkali metal halide electrolytethrough the porous diaphragm by positive means such as hydraulic flowand electro-osmotic pumping. This technique is referred to as thepercolating diaphragm method. This type of cell operation results in notonly a limitation of the concentration of the alkali metal hydroxideproduct, since the water content of the circulating aqueous electrolytesolution has a dilution effect and prevents concentration of thehydroxide, but also in retention of the impurities present in the brineinitially charged to the cell. Although the alkali metal hydroxidesolutions obtained with the diaphragm-type cell may be concentrated tomeet higher concentration requirements, the evaporation and purificationtechniques required are time consuming, inefficient and objectionablyexpensive.

In order to overcome the disadvantages of both the mercury anddiaphragm-type cells, membrane-type cells have been proposed forproducing alkali metal hydroxides. The permselective membranes used inthese cells are referred to as cationic since they permit the passage ofpositively charged ions. They are generally made from cation exchangeresins, usually ionogenic particles embedded or grafted into a fibermatrix or carrier. At low caustic concentrations such a cell designlimits the back migration of negatively charged hydroxyl ions and slowsdown the passage of water so that moderately high concentrations ofsolutions of alkali metal hydroxide are formed in the cathodecompartment, however, these cells require the addition of water to thecatholyte which causes lower current efficiency. Such membranes aredisclosed in U.S. Pat. No. 2,967,807, where their use in the productionof alkali metal hydroxide solutions is also shown. Various otherpermselective and so-called diaphragm-type membranes have been proposedin the prior art and such cationic membranes have solved the problem ofhalide ion exclusion and to some extent overcome the problem of the backmigration of the hydroxyl ions of the porous diaphragm cells as well asthe inclusion of objectionable impurities particularly alkali metalhalide, in the resultant product of such cells. However, the proposedmembrane cells also have limitations which have prevented theirwide-spread use such as lower current efficiency, structuraldegradation, low product concentration, high voltage and reducedoperating temperature requirements. The membranes are subject todegradation by the corrosive nature of the chemicals of the cells suchas chlorine, caustic and hypochlorite and are also degraded by higheroperating temperatures over rather short periods of time. For example,such prior membranes deteriorated after less than one thousand hours ofcontinuous operation. The increased expenses due to the frequentreplacement of such membranes has detracted from their use in obtainingimproved results over the porous and percolating diaphragm-type cell.The low current efficiencies found when the previous membrane cells wereused were caused by hydroxyl ion back-migration to the anode from thecathode chamber and its subsequent oxidation at the anode surface. Thelow voltage efficiencies were caused by the low permeabilities andheterogeneous gel characteristics of these membrane materials. Also highconcentrations of alkali metal hydroxide on the order of 50-56 percentare unobtainable with the use of the previous membrane cells, maximumproduct concentrations of only about 20 to about 40 percent have beenpreviously produced under optimum conditions.

SUMMARY OF THE INVENTION

It is the principal object of this invention to provide a method andapparatus for electrolytically producing high purity alkali metalhydroxide and chlorine in a membrane-type cell.

It is a further object of this invention to provide a method andapparatus for electrolytically producing high purity alkali metalhydroxide substantially free of alkali halide and other impurities in amembrane type cell with moderately high current efficiency operation.

It is an additional object of this invention to provide a process andapparatus for economically producing alkali metal hydroxide solution ofabout 30 to greater than about 55 weight percent concentrationsubstantially free of alkali metal halide and other impurities withoutthe necessity of additional purification steps and with minimum cellmaintenance requirements.

Other objects and advantages of the invention disclosed herein willbecome apparent to those skilled in the art from a reading of thefollowing specification and the appended claims, and by reference to theattached drawing.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic drawing of an electrolytic membrane cell usedin accordance with this invention and incorporating the hydratedselectively permeable membranes which have been found useful inaccordance with this invention.

FIG. 2 is a side view of the cell of FIG. 1 in assembled form.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings the electrolytic cell generally designated at9, is divided into an anode compartment 10, and a cathode compartment11, by membrane 12, which is held in position by cell half members 15aand 15c. The cell is also provided with electrolyte inlet 17, spentelectrolyte outlet 16, alkali metal hydroxide product outlet 18 andchlorine and hydrogen gas outlets 19 and 20, respectively. Adimensionally stable anode 13 and anode lead 13a, are positioned in theanode compartment 10 and cathode compartment 11 has disposed therein acathode 14 and cathode lead 14c. The anode may be of any suitableconfiguration such as a sheet or rod, flat or corrugated, rectangular orunsymmetrical. A foraminous sheet is preferred.

The dimensionally stable anode 13 is comprised of an electricallyconductive substrate with a surface coating thereon of a defect solidsolution of at least one precious metal oxide and at least one valvemetal oxide. In these solid solutions an interstitial atom of valvemetal oriented in the characteristic rutile valve metal oxide crystallattice host structure is replaced with an atom of precious metal. Thisdistinguishes the coating from mere physical mixtures of the oxides,since pure valve metal oxides are in fact insulators. Suchsubstitutional solid solutions are electrically conductive, catalyticand electrocatalytic.

Within the above-mentioned solid solution host structures the valvemetals include: titanium, tantalum, niobium and zirconium while theimplanted precious metals encompass platinum, ruthenium, palladium,iridium, rhodium and osmium. The mole ratio of valve metal to preciousmetal varies between 0.2-5:1, approximately 2:1 being presentlypreferred. The electrically conductive substrate may be constructed ofthe valve metals which are defined above as included in the solidsolutions. Titanium dioxide-ruthenium dioxide solid solution coatingsand titanium substrates are presently the preferred types of thesematerials.

If desired, these solid solutions may be "modified" by the addition ofother components which may either enter into the solid solution itselfor admix with same to achieve the desired result. For instance, it isknown that a portion of the precious metal oxide, up to 50%, may bereplaced with other metal oxides without substantial detrimental effecton the overvoltage.

The above-mentioned solid solution coatings and thermochemicaldeposition thereof on a substrate are described in more detail inpublished South African Patent Application No. 68/1834 and in BritishPat. No. 1,195,871.

Other dimensionally stable anodes constructed of an electricallyconductive substrate, e.g., a valve metal substrate, having a coating ofplatinum group metals and alloys thereof on at least a portion of thesubstrate may be used with good results. A platinum-iridium alloycoating on a titanium or tantalum substrate is particularlyadvantageous. The platinum group metals encompass the precious metals ofthe above-described solid solutions. Another type of suitabledimensionally stable anode consists of an anode bearing a coating of amatrix of tin oxide and antimony oxide in which a small amount ofplatinum group metal or platinum group metal oxide is dispersed, on atleast a portion of an electrically conductive substrate.

The cathode may be any suitable conductive material or metal capable ofwithstanding the corrosive catholyte cell conditions and which ischaracterized by low hydrogen overvoltage. A useful metal is generallyselected from the group of foraminous metals having a surface area andconsisting of stainless steel, nickel, cobalt, titanium, steel, lead andplatinum. The cathode may be in the form of a solid sheet or other solidmetal configuration or preferably it may be foraminous such as expandedmetal mesh or screen of high surface area. A foraminous stainless steelcathode with high surface area and good gas release characteristics isespecially preferred because it does not contaminate the caustic whendepolarized. In some cases it has been found that the cell of thisinvention can be operated more efficiently by using apolytetrafluoroethylene cloth mesh backing on the cathode side of themembrane, particularly in conjunction with a stainless steel cathode asthe hydrophobic fibers of the backing allow hydrated sodium ions to passto the cathode and prevent hydroxyl back-migration to the anode as thecloth fibers are not wetted by the cell solutions.

The membrane is preferably a film formed from a stable hydratedion-exchange resin which is a fluorinated copolymer having pendantsulfonic acid groups and contains a copolymer having recurringstructural units of the formula: ##STR1## and

(2) --CXX'--CF₂ --

wherein R represents the group ##STR2## in which

R' is fluorine or perfluoralkyl of 1 to 10 carbon atoms,

Y is fluorine or trifluoromethyl, and m is 1, 2 or 3,

n is 0 or 1; X is fluorine, chlorine, hydrogen or trifluoromethyl; andX' is X or CF₃ --CF₂)_(z) --

wherein z is 0 or an integer from 1 to 5; the units of formula (1) beingpresent in an amount from 3 to 20 mole percent.

The stable membranes corresponding to the above structural formulauseful in the practice of this invention have a water absorption ofabout 18% to about 38% in accordance with ASTM-D570 standard testingprocedures, and an equivalent weight of about 1000 to about 1300 and aresold by E. I. DuPont deNemours & Co., Inc. under the trade name XRPERFLUOROSULFONIC ACID MEMBRANES and more recently under the trademarkNAFION. The membranes and the method of preparation thereof aredescribed in more detail in British Pat. No. 1,184,321 and U.S. Pat. No.3,282,875. A technical bulletin published by E. I. DuPont deNemours &Co., Plastics Department, on Oct. 1, 1969 entitled XR PERFLUOROSULFONICACID MEMBRANES includes a detailed description of various physical andchemical properties of these membranes which illustrate the outstandingchemical thermal and oxidative stability thereof. The word stable asused herein in describing these membranes defines and encompasses theunique chemical thermal and oxidative stability of these membranes.

The invention will now be described with reference to the production ofchlorine and of a caustic soda product of high purity made by theelectrolysis of brine solution, but it is to be understood that theinvention is not restricted to production of sodium hydroxide but mayalso be utilized for making other alkali metal hydroxides by theelectrolysis of other aqueous metal halide solutions such as potassiumchloride. The process of the invention may be carried out on acontinuous basis by continuously introducing brine solution into theanode compartment of an electrolytic cell and initially introducingaqueous solution having a caustic soda content of about one to about 50weight percent or water into the cathode compartment of the cell,decomposing the brine solution by imposing a potential differencebetween the dimensionally stable anode and the cathode of said cellwhile maintaining the temperature of the solution in the cell at about35° C. to about 100° C., preferably about 65° C. to about 90° C., andthe pH at from 1.0 to about 5.5, preferably from about 2.0 to about 3.5,whereby the water and sodium ions are transported through apermselective cation exchange membrane film of the fluorinated copolymerhaving the previously described structural formula. The sodium ions passthrough said membrane into the cathode compartment from the anodecompartment along with water. A portion of the water molecules arereduced at the cathode to form hydrogen and hydroxyl ions andsubsequently sodium hydroxide solution of predetermined concentration inthe cathode compartment. The caustic solution is continuously removedfrom the cathode compartment generally without the introduction, from asource external of the cell, of additional water or caustic to saidcompartment after the initial inroduction. Although the continuousaddition of water or dilute alkali metal hydroxide to the catholyte froma separate source is not required during continuous operation, and nofurther such additions are generally made after the initial introductionsuch addition may be optionally desirable to adjust the alkali metalhydroxide concentration in the cathode compartment. The chloride ions inthe brine solution are attracted to the anode, oxidized and releasedfrom the anode compartment as chlorine gas. Hydrogen gas formed in thecathode compartment at the same time as the sodium hydroxide, is removedfrom the cathode compartment through a suitable vent. The membrane maybe of variable thickness, generally from about one mil to about 50 mils,but to obtain the higher concentrations of sodium hydroxide membranes of10 mils and 20 mils thickness, have been found to provide optimumresults at two distinctly different caustic concentration levels. Inusing a 10 mil membrane having a water absorption of about 25% causticconcentrations in the range of about 29% to about 44% by weightsubstantially free of sodium chloride are produced with high currentefficiency; with a 20 mil membrane having a water absorption of about25% to about 38%, even higher current efficiencies are obtained. Thepressure differential between the anode and cathode compartments is oneof the controlling factors in altering the amount of water transmittedosmotically from the anode compartment through the membrane. The watertransmission from the anode compartment may be depressed by maintainingpositive pressure in the cathode compartment thus increasing the causticconcentration. Thus the pressure differential between the anolyte andthe catholyte compartments, the thickness of the membrane and currentdensity can be controlled within established limits in order to obtaindesired caustic concentrations at satisfactory current efficiencies. Thetemperature in both the anode and catholyte compartments may vary widelyfrom about 35° C. to about 95° C. and within the range of about 70° C.to about 80° C. satisfactory results have been obtained. The spacebetween the membrane and each of the electrodes can be from about onetenth of a mil to about one inch for satisfactory results and isgenerally maintained from about 1/16 of an inch to about 5/16 inch. Foroptimum current efficiencies the anode to membrane gap preferably shouldbe about 1/16 to about 3/16 of an inch and the cathode to membrane gapfrom about 1/16 to about 1/4 of an inch.

One unique feature of the process of the invention which distinguishesit from the prior art is the fact that the process is operable withoutthe introduction of water or dilute alkali metal hydroxide solutiondirectly into the cathode compartment from a source external of the cellsince all the water which reacts in the cathode compartment istransported directly through the membrane as water of hydration inassociation with the migrating sodium ions or through electroosmotictransfer. Prior techniques for producing concentrated caustic solutionsother than the flowing mercury cathode process all require theintroduction of water or a dilute aqueous caustic solution into thecathode compartment of an electrolytic cell during electrolysis and suchaddition results in a loss of current efficiency. As noted above in thepractice of this invention water or dilute alkali metal hydroxidesolution is usually introduced to the cathode compartment only initiallyand added subsequently only if optionally desired.

The brine solution introduced into the anode compartment may vary widelyfrom about 100 g/l to about 325 g/l of NaCl concentration but preferablyis maintained at from about 160 g/l to the saturation concentration ofNaCl. Also excellent results may be obtained in the practice of thisinvention when either high purity or impure alkali metal halidesolutions are employed as the anolyte. During either batch or continuousoperation alkali metal halide is usually introduced to the anodecompartment as an aqueous solution of desired concentration. However,the alkali metal halide may obviously be introduced as a combination ofdry halide salt and water in amounts calculated to provide a desiredpredetermined concentration. In some cases after the initialintroduction of alkali metal solution to the anode compartment, theaddition of dry halide to the anode compartment may be desirable duringeither batch or continuous operation to maintain the concentration ofthe alkali metal halide anolyte at a desired level.

The cell of the present invention is constructed in two hollow sectionswhich allows the fluorinated copolymer cation exchange resin film to bepositioned and maintained in closely spaced relation to each electrodesurface by insertion of the membrane between the cell sections in a"sandwich" arrangement. This design may easily be modified to a filterpress system of series or parallel cells. The cell anode section 15asupports the dimensionally stable anode and is also provided with analkali metal halide electrolyte inlet, a spent electrolyte outlet and achlorine gas outlet in the hollow portion thereof. Electrical connectionmeans are also attached to the cell anode section for supplying currentto the anode. The hollow portion of the cell anode section 15c has acathode disposed therein and is also provided with an aqueous alkalimetal hydroxide solution outlet and a hydrogen gas outlet. When the cellanode and cathode sections are assembled with the membrane of 10 to 20mils in thickness disposed over the hollow portions of each section andthe assembly maintained in a predetermined fixed position by anysuitable means such as clamping devices, screws or bolts, and the like,good current efficiency and high yield of aqueous moderatelyconcentrated alkali metal hydroxide solutions substantially free ofalkali metal halide result from the electrolysis of saturated alkalimetal halide solutions. The material of construction of the cells may beany material resistant to or inert to the cell environment. Organicplastics such as polyvinyl chloride, polyvinyl fluoride, polypropyleneand inert resistant inorganic materials are useful for the cellcontainer sections. The unique construction of the cell which affordshigh current efficiency and excellent product yield also facilitates anarrangement for connecting a large number of such cells in series orparallel in a limited area. Significant savings are achieved through theresultant minimizing of floor space and the ease of replacing individualcells for maintenance without interruption of the entire cell bankoperation.

In order that those skilled in the art may more readily understand thepresent invention, specific examples are presented in Tables I to IIIbelow.

Table I shows the production of concentrated alkali metal hydroxidesolution substantially free of alkali metal halide with good currentefficiencies at variable voltages and current densities, membraneproperties and cell compartment temperatures. In Examples 1 to 5saturated brine solution was continuously introduced into the anolytecompartment of the type of electrolytic cell illustrated in the attacheddrawings and electrolyzed in accordance with the parameters shown in theexamples. In Example 6 an aqueous solution of potassium chloridecontaining 250 g/l of KCl was continuously introduced into the anolytecompartment and electrolyzed as specified in the example.

Table II illustrates the effect of varying the residence time of theanolyte, which directly corresponds to brine flow velocity, duringcontinuous introduction of a brine solution containing 280 g/l NaCl intothe anolyte compartment of the same type of cell as in Example I. Thetemperature in each of the anode and cathode compartments was maintainedat 82° C. and the caustic product obtained had a concentration of 370g/l. The brine flow velocity was controlled at a rate sufficiently lowto prevent formation of sodium chlorate and sufficiently high to avoidsevere depletion of the sodium chloride content of the brine. It will beobserved from the table that current efficiency improves with a decreasein brine velocity. Such results are entirely unexpected and surprisingas one skilled in the art would expect just the reverse to be the case.In other words, the current efficiency would be predicted by a skilledartisan to be improved by an increase in brine velocity. From the dataof the table it will be readily understood that control of the velocityof the brine continuously introduced into the anolyte compartment is animportant feature of this invention.

Table III shows the variation of caustic product concentration inrespect to a variation in feed brine concentration in the same type ofelectrolytic cell utilized in the examples of Table I. It will bereadily understood from Table III that the concentration of causticproduct increases directly with an increase in the concentration of theanolyte feed brine and that satisfactory caustic products can beobtained with the low concentrations of anolyte feed brine solutions.

                                      TABLE I                                     __________________________________________________________________________                   Membrane Properties                                                                       Temperature          Halide                             Current   Thickness                                                                           Water °C. in                                                                          Current                                                                             Product                                                                             Content of                    Example                                                                            Density   in    Absorption                                                                          Compartments                                                                           Efficiency                                                                          Concen-                                                                             Product                       Number                                                                             amps/in..sup.2                                                                     Voltage                                                                            Mills %     Anode                                                                             Cathode                                                                            %     tration g/l                                                                         g/l                           __________________________________________________________________________    1    1    3.46 10    25 76     86   84.5  539 NaOH                                                                            0.12 NaCl                     2    1    4.07 10    25 80     81   89.5  605 NaOH                                                                            0.42 NaCl                     3    2    5.4  20    25 65     75   85.6  760 NaOH                                                                            0.45 NaCl                     4    2    5.5  20    25 72     72   78.1  875 NaOH                                                                            0.08 NaCl                     5    2.9  5.12 10    38 --     92   74    310 NaOH                                                                            0.9 NaCl                      6    1.0  5.6  20    25 72     81   51.2  559 KOH                                                                             0.5 KCl                       __________________________________________________________________________

                                      TABLE II                                    __________________________________________________________________________               Percent Loss                                                                  of Current                                                         Residence  Efficiency                                                                            Membrane Properties                                             Time of                                                                             Due to Sodium                                                                         Thickness                                                                           Water Current                                        Example                                                                            Anolyte                                                                             Chlorate                                                                              in    Absorption                                                                          Density                                        Number                                                                             (Minutes)                                                                           Formation                                                                             Mils  %     Amps/in..sup.2                                 __________________________________________________________________________     7   230   15      10    25    1.0                                             8   115   17      "     "     "                                               9   77    19      "     "     "                                              10   58    21      "     "     "                                              11   46    23      "     "     "                                              12   38    25      "     "     "                                              13   33    27      "     "     "                                              14   29    29      "     "     "                                              15   23    33      "     "     "                                              16   12    40      "     "     "                                              __________________________________________________________________________

                                      TABLE III                                   __________________________________________________________________________                   Membrane Properties                                                                       Temperature    NaOH  NaCl                               Current   Thickness                                                                           Water °C. in                                                                          Current                                                                             Product                                                                             Content of                    Example                                                                            Density   in    Absorption                                                                          Compartments                                                                           Efficiency                                                                          Concentra-                                                                          Anolyte feed                  Number                                                                             amps/in..sup.2                                                                     Voltage                                                                            Mils  %     Anode                                                                             Cathode                                                                            %     tion g/l                                                                            Brine g/l                     __________________________________________________________________________    17   1.0  3.91 20    25    75  75   67.6  315   126                           18   "    3.83 "     25    "   "    65.0  315   133                           19   "    4.00 "     25    "   "    72.6  332   160                           20   "    4.38 "     25    "   "    77.0  353   160                           21   "    4.50 "     25    "   "    80.5  350   161                           __________________________________________________________________________

From the above examples it is obvious that concentrated aqueous sodiumhydroxide and potassium hydroxide solutions substantially free of sodiumand potassium chlorides respectively, can be obtained by the practice ofthe present invention at variable temperatures, current densitiesand/alkali metal halide electrolyte concentrations. High purity productsof predetermined concentrations are obtained at high currentefficiencies when hydrated membranes of specified structure, propertiesand thickness are included in the cell arrangement.

Although the invention has been described with reference to certainpreferred embodiments thereof, it is not to be so limited since changesand alterations may be made therein which are within the full andintended scope of the appended claims.

We claim:
 1. An electrolytic cell for the production of aqueous alkalimetal hydroxide solution substantially free of alkali metal halide andother impurities comprising(1) a first hollow cell section havingdisposed therein a dimensionally stable anode, an electrolyte inlet, aspent electrolyte outlet, a halogen gas outlet and electrical connectionmeans for supplying current to said anode; (2) a second hollow cellsection having disposed therein a metal cathode, an outlet for theaqueous solution of alkali metal hydroxide product, an outlet forhydrogen gas and electrical connection means for conducting current fromsaid cathode; and (3) a hydrated fluorinated copolymer cationic exchangeresin film membrane having pendant sulfonic acid groups or derivativesthereof arranged between said hollow cell sections closely spacedbetween a surface of each said anode and said cathode and held in fixedjuxtaposed position by said hollow cell sections, said film membranehaving the structural formula: ##STR3## and

     --CXX'--CF.sub.2 --                                       (2)

wherein R represents the group

    --CF--CF.sub.2 --O--CFY--CF.sub.2 O).sub.m

in which R' is fluorine or perfluoralkyl of 1 to 10 carbon atoms, Y isfluorine or trifluoromethyl, and m is 1, 2 or 3, n is 0 or 1; X isfluorine, chlorine, hydrogen or trifluoromethyl; and X' is X or CF₃--CF₂)_(z) wherein z is 0 or an integer from 1 to 50; the units offormula (1) being present in an amount from 3 to 20 mole percent.
 2. Thecell of claim 1 wherein the dimensionally stable anode consistsessentially of a valve metal substrate having a coating of a solidsolution of at least one valve metal oxide and at least one preciousmetal oxide on at least a portion of its surface.
 3. The cell of claim 2wherein the valve metal substrate is titanium having a coating of asolid solution of ruthenium dioxide and titanium dioxide on at least aportion of its surface.
 4. The cell of claim 1 wherein the dimensionallystable anode consists essentially of a valve metal substrate having acoating of a platinum group metal or alloy thereof on at least a portionof its surface.
 5. The cell of claim 4 wherein the coating is aplatinum-iridium alloy.